Recently, the TDMA communication system has been adapted to the field of satellite communications, and accordingly new techniques are required for detecting a fault of the device itself in a transmission apparatus including a transmission frequency converter or a power amplifier.
One example of a prior art fault detector used in a FDM-FM system is shown in FIG. 1. The signal output from the modulator 1 is input to the transmission frequency converter 2. The frequency-converted signal is amplified by the power amplifier 3, and it is transmitted from an antenna 4. In such a device, correct signal transmission is detected as follows:
The output of the power amplifier 3 is dividedly taken out by a directional coupler 5, and it is detected by a detector 6 or a like power sensor. The detected output is amplified by a d.c. amplifier 7, and a window comparator 8 or the like detects whether the level of the output of the d.c. amplifier 7 is within a permissible range (decided by an upper and a lower predetermined level). Thus, the abnormality of the device is detected. By providing a similar circuit at the output side of the transmission frequency converter 2, it is possible to detect abnormality in the path to the transmission frequency converter 2.
In conventional FDM-FM systems such a method for detecting abnormality can be used because the power of transmission output is usually constant. However, such a detecting circuit, cannot operate as an abnormality detector in transmission system of so-called burst mode signals because the transmission output is switched ON/OFF constantly, thereby making the output level non-constant in these systems. Furthermore, in TDMA communication systems various types of burst mode signals which have different lengths and intervals are transmitted. Accordingly, when the detector shown in FIG. 1 is used in such a transmission system, it is necessary to make the time constant of the above-described circuit large so as to detect the average power of the transmission output. But, such construction cannot operate satisfactorily.